As selective call network coverage areas grow to meet consumer demand in larger metropolitan areas, selective call network service providers must add additional transmitters to increase coverage area. However, interference between signals sent from the several transmitters cause difficulty in reception. This interference occurs in those areas where a selective call receiver can receive transmissions from two or more transmitters. As shown in FIG. 1, a conventional paging terminal (controller) 102 provides a signal to four transmitters 110A, 110B, 110C, and 110D. Each transmitter has an associated coverage area 106A, 106B, 106C, and 106D into which the signal from the controller is broadcast. Due to the difference in transmission path lengths and switching equipment, the transmission of the signal from one transmitter (110B for example) may be delayed with respect to the transmission of the signal from another transmitter (such as 110A). It is this delay that causes interference in overlapping coverage areas 108, because of the difference in arrival times of the signals from different transmitters.
To overcome the signal interference due to staggered transmitting times, some communication systems provide simultaneous transmission from the transmitters 110A-D. This process is commonly referred to as simulcast. Simulcast is a reliable method of achieving wide area coverage for one-way (paging) and certain other types of two-way communications. Obviously, simulcasting is not appropriate for all paging systems. However, for wide area coverage, simulcasting offers operational advantages not available in other conventional paging systems. For example, more selective call receivers (pagers) can be accommodated per channel, because obstruction losses due to buildings etc. are considerably reduced by multiple transmitter configurations.
One known simulcast system involves placing large coils(called equalization coils) in the transmission path from the terminal to each transmitter. By manually varying the amount of coil inserted in the transmission path the reception in the overlapping coverage area 108 can be improved. Regrettably, however, the equalized coils do not take into affect the variations in the length of the transmission path when a Public Switch Telephone Network PSTN is utilized. As is well known in the art, a PSTN service provider can route a call in any manner, at the providers option, as long as the call originates and ends at the required locations. Moreover, random intercall rerouting may also insert additional equipment into the transmission path further varying the time the signal arrives at the transmitter.
Another known simulcast solution, allows for presetting the delays at each transmitter and governing the transmission of the signals from the transmitters by accurate clocks, thereby simultaneously transmitting the signals. Regrettably, such a system is extremely costly due to the clocks.
In a conventional simulcast synchronization phase, the simulcast system transmits a known signal to measure delays between each base station and the controller to synchronize the simulcast transmissions. The selective call receivers within the system typically cannot recognize the synchronization signals. Unfortunately, the selective call receivers, during the synchronization phase will try to decode the random patterns in the synchronization sequence, which often results in "falsing". Falsing occurs when a selective call receiver incorrectly decodes an address of another device as its address. Also, the synchronization signal causes the system to spend a longer time in the synchronization phase, because the system has to re-format the signals differently in the paging mode than in the synchronization mode. This increase time translates in an unfavorable cost increase to the consumers of the paging system, because the longer synchronization time results in additional distributed charged to users.
Thus, what is needed is a simulcast system capable of synchronizing the transmission of signals from the transmitters while reducing the cost to the users and the potential of "falsing" during the synchronization phase.